The present invention relates to solid-state image detectors and its purpose is to eliminate the variations in their sensitivity, especially those due to temperature variations.
In these image detectors, the acquisition of an image takes place with the aid of several photosensitive spots each formed from a photodiode and a switch. The photosensitive spots are produced with the aid of techniques for the thin-film deposition of semiconductor materials such as hydrogenated amorphous silicon (aSiH). These photosensitive points, arranged in the form of a matrix or linear array, make it possible to detect images contained within visible or near-visible radiation. The signals that are produced are then digitized so as to be able to be stored and processed easily.
These arrangements of photosensitive spots find one particularly advantageous application in the medical field or the field of industrial inspection, in which they detect radiological images. All that is required is to cover them with a scintillator and to expose the latter to X-radiation carrying a radiological image. The scintillator converts the incident X-radiation into radiation in the band of wavelengths to which the photosensitive spots are sensitive.
There are now large photosensitive matrices which may have several million photosensitive spots. FIG. 1 shows a matrix image detector of the known type. It has only nine photosensitive spots in order not unnecessarily clutter up the figure. Each photosensitive spot P1 to P9 is formed from a photodiode Dp and an element having a switch function Dc represented in the form of a switching diode. It would have been possible to choose a transistor as the element having a switching function. The photodiode Dp and the switching diode Dc are connected together in a head-to-tail arrangement.
Each photosensitive spot P1 to P9 is connected between a row conductor Y1 to Y3 and a column conductor X1 to X3. The row conductors Y1 to Y3 are connected to an addressing device 3 known as a driver. There may be several drivers 3 if the matrix is of large size. The addressing device 3 generally comprises shift registers, switching circuits and clock circuits. The addressing device 3 raises the row conductors Y1 to Y3 to voltages which either isolate the photosensitive spots P1 to P3 connected to the same row conductor Y1 from the rest of the matrix or turn them on. The addressing device 3 allows the row conductors Y1 to Y3 to be addressed sequentially.
The column conductors X1 to X3 are connected to a read device CL.
During an image record phase, during which the photosensitive spots P1 to P9 are exposed to information to be picked up and are in a receiving state, that is to say their reverse-biased photosensitive diodes Dp and switching diodes Dc each constitute a capacitor, there is a build up of charges at the junction point A between the two diodes Dp, Dc. The amount of charge is approximately proportional to the intensity of the signal received, whether this is very intense illumination, provided that the photosensitive diodes remain in the linear detection range, or darkness. There then follows a read phase, during which a read pulse is sequentially applied to the row conductors Y1 to Y3, which read pulse turns on the photodiodes Dp and makes it possible for the charges accumulated in the column conductors X1 to X3 to drain away to the read device CL and for them to be integrated.
A read device CL will now be explained in greater detail. It consists of as many read circuits 5 as there are column conductors X1 to X3 and these read circuits are of the charge-integrating circuit type. Each photosensitive spot is connected to a read circuit 5. Each charge-integrating circuit is formed by an operational amplifier G1 to G3 mounted as an integrator by means of a read capacitor C1 to C3. Each capacitor C1 to C3 is mounted between the negative input of the operational amplifier G1 to G3 and its output S1 to S3. Each column conductor X1 to X3 is connected to the negative input of an operational amplifier G1 to G3. The positive input of each of the operational amplifiers G1 to G3 is taken to a constant input reference voltage VR, which sets this reference voltage on each column conductor X1 to X3. Each operational amplifier G1 to G3 comprises a resetting switch I1 to I3 mounted in parallel with the capacitor C1 to C3.
The outputs S1 to S3 of the integrating circuits are connected to a multiplexing device 6 which delivers, as a series, signals corresponding to the charges which were integrated by the charge-integrating circuits. In the read phase, these signals correspond to the charges accumulated over an integration time by all the photosensitive spots of the same row. The signals delivered by the multiplexing device 6 are then digitized in at least one analog-digital converter 7, the digitized signals output by the analog-digital converter 7 translating the content of the image to be detected. These digitized signals are sent to a management system 8 which can store, process and display them.
It has turned out that the sensitivity of such detectors varies, which results both in local and overall variations in the brightness of the image detected. There are several causes of the variations in sensitivity. Firstly, there is a spatial variation and secondly a thermal variation. This means, on the one hand, that two photosensitive spots of the detector cannot give the same response when they are exposed to precisely the same luminous flux and, on the other hand, that a photosensitive spot exposed to the same luminous flux does not give the same response at 25xc2x0 C. as it does at 35xc2x0 C. These discrepancies are partly due to the semiconductor components constituting the photosensitive spots, which components do not all come from the same manufacturing batch, and partly to the scintillator material used in radiology. This results in images with nonuniform areas which should not be there and which become increasingly pale as the temperature increases.
Although it is known how to overcome the spatial variation in sensitivity by making a correction to the image with a so-called gain image, it is not possible to use the gain image to overcome thermally induced variations in sensitivity.
The gain image is an image taken with a calibrated uniform illumination in the absence of a subject or object to be examined. This gain image allows the spatial variations in sensitivity to be properly corrected, since with a uniform illumination the image should be uniform. This gain image is produced with a very low frequency, of the order of one year. The signals delivered by the photosensitive spots when the gain image is being read are stored in the management device 8 and then serve to correct, for spatial inhomogeneity in the sensitivity, any useful image.
This method cannot be used to overcome thermally induced variations in sensitivity: this would require producing gain images in synchronism with the temperature variations, which would significantly increase the frequency at which gain images are recorded. This is not compatible with the manner in which operators use such image detectors.
The present invention proposes the use of a gain image or a quasi gain image matched to the ambient temperature in order to obviate variations in the sensitivity of the image detector, especially thermally induced variations, but this gain image is not simply recorded just before making the correction, in order to be matched to the ambient temperature, but it is generated from a calculation resulting in the determination of the ambient temperature.
To achieve this, the present invention provides a method for temperature compensation of the sensitivity of an image comprising photosensitive spots, each with a photodiode, these being connected to read circuits, characterized in that the photosensitive spots are divided into detecting photosensitive spots, capable of detecting an image when they are exposed to information carrying the image and are sensitive to this information, and into blind spots protected from the information, and in that it consists:
when the photosensitive spots are taken to a reference temperature, in calculating an average leakage current in the photodiodes of the blind photosensitive spots and in generating an average from the signals delivered by the blind photosensitive spots during a read operation;
when the photosensitive spots are taken to an ambient temperature to be determined, in generating another average from the signal delivered by the blind photosensitive spots during another read operation;
in calculating the ambient temperature from the average leakage current and from the distance between the two averages;
in generating a gain image or a quasi gain image matched to the ambient temperature; and
in correcting an image recorded at the ambient temperature with the gain image or the quasi gain image.
Preferably, the signals delivered for generating the first average and for generating the average at the ambient temperature to be determined correspond to charges integrated over a first integration time approximately equal to the nominal integration time of the image detector.
To calculate the average leakage current at the average reference temperature, it is possible to generate, at the reference temperature, a pair of averages from the signals delivered by the blind photosensitive spots over two different integration times and to make a calibration using the pair of averages.
One of the averages of the pair is advantageously the first average. The other average of the pair corresponds to charges integrated over an integration time longer than the nominal integration time of the image detector.
The gain image matched to the determined ambient temperature may be generated from a series of gain images stored beforehand in a memory device, each of them being recorded at a different particular temperature, all of these particular temperatures forming a range of temperatures at which the image detector is likely to operate.
The quasi gain image matched to the determined ambient temperature may be generated, advantageously, from a base gain image recorded at a base temperature and corrected with the aid of a coefficient of variation of the base gain image as a function of temperature and taking into account the difference between the determined ambient temperature and the base temperature.
The base temperature may be the reference temperature. Advantageously, the averages may be generated from black images.
The present invention also relates to an image detector for implementing the compensation method, comprising photosensitive spots each with a photodiode, these photosensitive spots being connected to read circuits. These photosensitive spots are divided into detecting photosensitive spots, capable of detecting an image when they are exposed to information carrying the image, and into blind spots protected from the information. The detector comprises means for calculating the averages from the signals delivered by the blind photosensitive spots, means for calculating the average leakage current in the diodes of the blind photosensitive spots, means for calculating the ambient temperature from the average leakage current and from the distance between averages, means for generating the gain image or the quasi gain image from the calculated ambient temperature and means for correcting an image recorded at the ambient temperature with the gain image or the quasi gain image.
The means for calculating the average leakage current receive the averages of the signals delivered by the blind photosensitive spots in digital form. The means for generating the gain image may comprise a memory device containing one or more gain images, each corresponding to one temperature.
The means for generating the quasi gain image may comprise a memory device containing a base gain image recorded at a base temperature and a coefficient of variation of the base image with temperature.
It is preferable for the blind photosensitive spots to be connected to outermost portions of conductors to which the detecting photosensitive spots are connected. The blind photosensitive spots are covered with a material opaque to the information received by the detecting photosensitive spots, this material being in particular black paint.
The detecting photosensitive spots are covered with a scintillator material which converts X-radiation into radiation to which they are sensitive, the blind photosensitive spots being covered with an X-ray-opaque material, such as lead.
The material opaque to the information lies between the X-ray-opaque material and the blind photosensitive spots.